RISC-V: Provide a cleaner raw_smp_processor_id()

I'm not sure how I managed to miss this the first time, but this is much
better.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
[Atish: code comment formatting and other fixes]
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>